The present invention relates to secondary containment systems for above-ground tanks for holding hazardous and non-hazardous materials such as petroleum products, solvents, paint thinners or any other liquid which would cause an environmental problem if released into the ground. It is particularly useful for flammable liquids; that is, those with a flashpoint below 100.degree. F.
Recently-adopted regulations of the Environmental Protection Agency require that above-ground tanks be provided with a secondary containment device to catch and retain any spills or leaks from the primary above-ground tank. Known products for this purpose have taken the form of a large open-topped tub, usually called a dike, into which the primary tank is placed. While this arrangement permits leaks and overfills to be caught and prevented from contaminating the ground, such leaks and overspills are exposed to the atmosphere and may be contaminated by rain, snow or trash thrown into the open tub. If the leaked or overfilled product is otherwise usable, such contamination would render it not usable, such that it must be treated as hazardous waste, rather than valuable, usable product. For waste products, such contamination may make the purification, recycling or other handling of the products more difficult.
In addition, exposing such leaked or spilled products to the atmosphere in an open tank may contribute to atmospheric pollution, since many such products are volatile and will evaporate from an open-topped dike, thereby contributing to smog and other air pollution. Furthermore, spilled volatiles held in an open-topped dike pose an explosion hazard. In one case, in which gasoline leaked from a tank into an open-topped dike evaporated, the vapor travelled in the prevailing wind a mile to a mobile home park, where a water heater pilot light caused an explosive ignition. The flame travelled the route back to the storage tank on a trail of vapor, engulfing the tank in flames. Even when evaporation does not lead to such tragic circumstances, economic loss due to the simple loss of product through vaporization is an undesirable feature of open-topped dike secondary containment devices. The safety aspect of the storage of flammable materials has long been regulated. Materials are classed as flammable by the National Fire Protection Association if they have a flashpoint below 100.degree. F. Examples are gasoline and kerosene. Materials with a flashpoint at or above 100.degree. F. such as motor oils, transmission fluids, hydraulic oil, paraffin or synthetic base lubricating oils, No. 2 fuel oil, No. 4 fuel oil and cutting oil are classed as combustible materials since they have flashpoints above 100.degree. F. The lower flashpoimts of flammable materials makes them more likely to ignite with explosive or burning results than combustible materials. Also, the lower flashpoints usually mean that the vapor pressure of a flammable material will be higher at a given temperature than for a combustible material. These two attributes of flammable materials, capability of exploding and increased vapor pressure, have caused safety agencies such as the National Fire Protection Association and Underwriters Laboratories, Inc. (UL) to adopt well-recognized standards applicable to tanks for holding flammable materials. That is, in order to be approved for such use by those agencies (and regulatory authorities such as OSHA, U.S.D.O.T., and local building codes which have adopted NFPA and UL standards as their own), the tanks must be tested to withstand 5 to 7 psig, among other requirements. Thus, a tank for holding flammable liquids must meet those standards in order to be commercially practical. The standards involved are NFPA Standard 30 and UL 142.
Accordingly, there is an need in the art for a secondary containment device for above-ground primary holding tanks for both raw materials and hazardous waste, which overcomes the problems of contamination of product, undue exposure of the atmosphere to polluting or hazardous volatiles and vapor spilled product.